This invention relates generally to pneumatic motors, and more particularly concerns a piston and cylinder device in which a pneumatic valve automatically causes the piston to reciprocate by alternately directing flow of air to and from each side of the piston.
Reciprocating piston and cylinder devices, commonly used as pumps and motors, are generally either single acting or double acting. In single acting piston and cylinder devices, fluid under pressure is selectively directed to only one side of the piston in a forward stroke and means such as a return spring return the piston to its original position. In double acting piston and cylinder devices, fluid under pressure is selectively directed to one side of the piston to drive it in a forward stroke and alternately to the opposite side of the piston to drive it in a return stroke. Usually the control of the pressurized fluid is performed by a main directional valve that alternately directs the fluid to one of two supply passages connected to opposite ends of the cylinder. The side of the piston that is not being pressurized is exhausted to the environment. Air is often the pressurized fluid, and such devices are generally referred to as air motors.
Because single acting piston and cylinder devices rely on mechanical components such as springs and poppet valves actuated by mechanical toggles or trips to function, they are subject to wear. In addition to requiring maintenance, these devices create noise when the toggles are contacted.
Double acting piston and cylinder air motors typically include an air valve assembly within an independent air valve cylinder. The air valve assembly may include a spool-shaped member as the main valve that controls the direction of airflow. At any given time, the spool is positioned towards either one end of the air valve assembly or the other. Two pilot pistons control the position of the spool. Some devices require that the main piston, pilot pistons, or both have differential cross-sectional surface areas in order to create the differential force required to shift the spool. This significantly increases the overall manufacturing cost of the device.
In devices with two identical pilot pistons, the pilot pistons respond to pressurized air flowing through passages originating from pilot ports in the main piston cylinder near the end of the main piston stroke. When the main piston moves past a pilot port to place the pilot port on the high-pressure side of the main piston, a pilot piston will move and shift a corresponding pilot valve. Pilot valves, reciprocally disposed in a through-bore in the spool, selectively vent air chambers at opposite ends of the spool. This results in shifting the spool to direct pressurized air to the opposite side of the main piston, reversing the direction of movement of the main piston.
Conventional double acting devices often malfunction at the cold temperatures incidental to use of compressed gasses. The usual cause of the malfunction is moisture in the air freezing around the spool and the pilot valves. The ice causes the air motor to slow and occasionally stop by clogging passages. Such malfunctioning may occur within five minutes of continuous use. The device cannot be used again until the ice melts. Conventional devices also have airline oil lubrication. This type of lubrication, which is standard industry practice, washes out the grease in the air motor, causing premature failure.
Pneumatically controlled and driven piston and cylinder devices are often used as the motive force for pumping of viscous fluids. For example, such devices are used in manufacturing facilities and commercial automotive maintenance shops to deliver grease, motor oils, gear oils, hydraulic oils, and automatic transmission fluid from original refinery drums or tanks to the location of use.
Noise attenuation is a concern with air motors because the air exhausting at high velocity from the motor can produce excessive sound levels, often in environments such as manufacturing facilities and maintenance shops where workers are in the immediate vicinity on a prolonged basis. Mufflers are the most popular method of noise attenuation. Mufflers are most often canister-type devices that are mounted externally to the air motor. The muffler receives the exhaust air from the motor and expands the air, thereby reducing the air velocity before discharging the air to the environment. One type of muffler design includes an expansion chamber that requires the muffler to be quite large, sometimes as large as the air motor itself, increasing the cost of the air motor. More complex designs include filtering or baffling systems that also increase the cost. Compact mufflers are generally less effective in attenuating noise than is desirable.
Accordingly, there is a need for a pneumatic motor that functions well under a variety of environmental conditions, has durable parts and a long life, is relatively quiet and compact, and is relatively low in cost to manufacture.
Accordingly, it is an object of the present invention to provide a pneumatic motor that operates reliably for extended periods.
Another object of the present invention is to provide a pneumatic motor that is durable and has low maintenance requirements.
Still another object of the present invention is to provide a pneumatic motor that runs quietly.
A yet further object of the present invention is to provide a pneumatic motor that is compact and relatively inexpensive to manufacture.
According to the present invention, a double acting pneumatic motor is provided that comprises a cylinder, within the cylinder a piston that divides the cylinder into two cavities that vary in volume with movement of the piston, a source of fluid under pressure, and a valve assembly for alternately directing fluid to one cavity while exhausting the other cavity to make the piston reciprocate.
The valve assembly resides in a housing with a cylindrical inside surface, and includes a substantially cylindrical spool, or main valve member, that moves axially between two positions. The housing is in continuous fluid communication with each cavity in the cylinder by way of two passages. The spool alternately directs pressurized air to one passage and exhausts the other passage to the environment, and has a central through-bore that at least a part of which continuously communicates with the pressurized fluid source. The spool has at least one portion with an enlarged diameter in between a smaller diameter portion at each end.
Two variable volume fluid pressure chambers are on opposite sides of the enlarged diameter portion of the spool. A pilot valve substantially within the central bore of the spool also moves axially between two positions that alternately pressurize or exhaust the pressure chambers adjacent to the enlarged diameter portion of the spool. The position of the pilot valve determines the position of the spool, and the position of the spool determines which cavity in the cylinder is pressurized, which cavity is exhausted, and the direction of motion of the piston.
The present invention further comprises a pilot piston at each end of the pilot valve. The pilot pistons are responsive to pressurized air from the cylinder, and cause the pilot valve to shift between its two positions.
A frame within the housing is provided in and around which the spool, pilot valve, and piston valves move. The frame comprises two exhaust adapters and two pilot adapters. The valve assembly parts are exemplarily made of acetal resin.
Also according to the present invention, a pneumatic motor may be provided with a cover for attenuating sound from the exhaust of the pneumatic motor. The cover includes a curved portion that is radially spaced from the curved outer surface of a cylindrical member outside the valve assembly to form an exhaust flow path. The curved portion directs exhaust flow in a substantially tangential direction along the surface of the cylindrical member. The cover may further comprise an expansion chamber and a muffler. Where the cylinder and valve assembly are incorporated into one substantially cylindrical body, the body serves as the curved surface over which exhaust flow is directed. The cover, expansion chamber, and exhaust flow path may all be substantially the same length as the body.
The present invention features a spool and pilot valve that each have two positions, combining to create a four-way valve. The spool, exhaust adapters, pilot adapters, and housing define seven pressure chambers. In one embodiment, two of the chambers are continuously pressurized, three are continuously exhausted, and two are alternately either pressurized or exhausted resulting in a force on the spool that impels the spool to move. Alternatively, in another embodiment, three of the chambers are continuously pressurized, two are continuously exhausted, and two are alternately either pressurized or exhausted resulting in a force on the spool that impels the spool to move. The components of the valve assembly are arranged to create chambers, ports, or passages that direct the flow of pressurized air to and from each side of the piston. Movement of parts changes the available passages in which air can flow by realigning the various passages through the parts and the chambers that are formed by the recessed areas of the parts. The cover improves the diffusion of exhaust and sound attenuation by taking advantage of a phenomenon known as the Coanda effect.
The pneumatic motor has only four moving parts to alternately direct pressurized fluid to the piston chambers. No mechanical levers or springs are required. The piston reciprocates rapidly until the flow of pressurized air is stopped. The pneumatic motor minimizes or eliminates occasions when ice forms around the pilot valve, which can cause the motor to malfunction, such as the main valve stopping between its two positions and allowing air to flow directly from supply to exhaust, with no effect on the piston. Little maintenance is required. The pneumatic motor is also compact and operates relatively quietly.